Gratings with a large aspect ratio, that is to say a large ratio between the structure height and the width of the grating gaps, are known. Herein, a large aspect ratio is understood to mean a value greater than 10. However, it should be noted that gratings with an aspect ratio of up to 50 are also known. Given a prescribed grating width, a large aspect ratio requires, in particular, a large structure height, that is to say grating gaps that are as deep as possible. Such gratings are used in e.g. phase-contrast X-ray imaging for determining the phase-shift of an X-ray beam as it passes through an examination object.
Such gratings are usually produced by way of a lithography method, in particular by means of the so-called LIGA method. This method is particularly suitable for producing gratings with large structure heights—up to 3 mm—and the smallest possible lateral dimensions—up to 0.2 μm. This allows the realization of large aspect ratios. Herein, a radiation-sensitive layer, the resist, is illuminated using the shadow cast by a working mask, as a result of which an exact image of the working mask is transferred into the resist. This primary structure represents an exact negative impression of the subsequent grating. Subsequently, the irradiated regions are removed by chemical means. By way of example, a plastic such as PMMA or a negative resist such as SU-8 can be used as a resist. During the irradiation by energetic and parallel X-ray radiation in particular, very high structures with almost perpendicular and very smooth sidewalls can be produced compared to irradiation by UV radiation, and this is useful for the required high aspect ratio.
If these primary structures are generated on a metallic start layer, the structures exposed after the development process can be filled by means of electroplating with different metals or alloys forming the actual grating. After the growth of the grating material, the primary structure is removed and merely the grating structures remain. If the grating material is grown significantly over the structure height of the resist, an interconnected, stable plate is obtained—the grating base—and it supports the grating structures. Overall, a multiplicity of different metals, alloys, ceramics and plastics can be used as grating materials in this method.
The publication “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources” by F. Pfeiffer et al., in Nature Physics (2006) advanced online publication, describes the options for phase-contrast X-ray imaging using an incoherent X-ray source. In order to implement these imaging systems, the production of grating structures with a recurrence in the region of 2 μm and structure heights of at least 100 μm is necessary. These requirements in respect of the height of the absorbing grating structures and the mechanical stability thereof currently present large production-technical problems.
The publication “Fabrication of, high aspect ratio submicron gratings by soft X-ray SU-8 lithography” by E. Reznikova et al., in Micro. Syst. Techn. (2008), describes a method by means of which gratings can be produced with an aspect ratio of greater than 50. Here, the production of virtually defect-free grating structures with a width of up to 1.2 μm and a height of 60 μm was demonstrated. Here, the grating structures consist of the recurring alternating grating webs and grating gaps, wherein a plurality of filler beams are respectively arranged in the grating gaps in order to stabilize the grating webs. Here, the spacing between adjacent filler beams in a grating gap is irregular. This stabilization by the filler beams is required particularly in the high grating structures required for using the gratings in phase-contrast X-ray imaging because the influence of the surface effects, i.e. the capillary forces, acting in the grating gaps to the left and right of a grating web increase strongly as the height increases.
However, this article also describes that structure heights of more than 60 μm lead to bending of the grating webs. As a result of the irregular spacings between the filler beams, capillary forces of different strengths act in the grating gaps. This non-equilibrium of forces leads to a bending of the grating webs between two filler beams. Accordingly, this bending then limits the aspect ratio and such mechanically unstable or bent gratings cannot simply be used in phase-contrast examinations.